The recent development of the microstructured optical fiber, in which a high index core region is surrounded by cladding having a mix of silica and air, offers new fiber properties by virtue of the large refractive-index contrast that exists between glass and air.
Reference is made to Section 4.3.6 of Takanori Okoshi, “Optical Fibers”, Academic Press (1982) for discussion of the modes of an optical fiber. As described therein, HE11 mode is the fundamental mode and the others are the higher-order modes.
A properly designed microstructured optical fiber can support numerous transverse spatial modes that are essentially decoupled from one another, as described in U.S. Pat. No. 6,400,866. In one disclosed embodiment, an optical pump propagates in one transverse mode, and an optical signal in another. A microstructured optical fiber, which can support several decoupled transverse modes, can be used to phase match nonlinear optical processes such as second harmonic generation and four-wave mixing. An optical pump and an optical signal are coupled into respective transverse modes of a microstructured optical fiber having decoupled transverse modes. As a result, a third signal is generated, which may be, for example, a sum or difference frequency signal, a second or third harmonic signal, signals at frequencies above and below the pump frequency. While this patent suggests the usage of higher-order modes of a microstructured optical fiber for nonlinear optical processes, there is no suggestion of how, or recognition that, optical signal regeneration can be obtained free from frequency conversion.
A paper entitled All-Optical Data Regeneration Based On Self-Phase Modulation Effect, P. V. Mamyshev, ECOC'98 p.475 (1998) describes feeding a pulsed optical signal of return-to-zero (RZ) format into a nonlinear medium, where self-phase modulation (SPM) broadens the spectra of the signal. The output from the nonlinear medium passes through an optical filter having a central frequency offset from that of the original signal. By this configuration, the noise in “zeros” and the amplitude fluctuation in “ones” of the signal is suppressed, so that the signal is regenerated. However, the regenerated signal must have a frequency different from that of the original signal, so that the complexity of optical system increases by the incorporation of the regenerating module. Also, it is difficult to incorporate this regenerating module into existing optical systems.
The need thus exists in the prior art for an optical module of simple configuration that provides an increase in signal gain with increase of signal input power and that can phase match pump radiation and signal.